Coal-fired power plants and other industrial facilities produce large amounts of coal ash, commonly known as fly ash. Fly ash is a waste product of the coal burning process, and must be disposed of properly due to environmental regulations. The large amounts of fly ash produced yearly in the United States, therefore, pose a significant disposal burden for industry.
Although fly ash may be characterized as a waste product by some industrial facilities, a beneficial use of fly ash as an additive for concrete has been well established, provided the fly ash does not contain excessive amounts of residual carbon left over from the coal burning process. Large volumes of low carbon fly ash may be used as a pozzolonic material in concrete mixtures, both as a partial Portland cement replacement and as a mineral additive. The use of fly ash increases the quality of the product by adding strength, increasing sulfate resistance, and enabling more economical concrete production. However, the use of fly ash containing excessive amounts of carbon is unacceptable.
Many coal-fired power plants produce fly ash with low amounts of residual carbon, referred to as Loss-On-Ignition or LOI. However, numerous power plants and other facilities produce fly ash with LOI levels above that which should be used in concrete. This high LOI fly ash can be converted to a saleable product, if the amount of residual carbon is reduced to or below acceptable carbon levels. Typically, LOI levels of about 3% or below are acceptable for fly ash to be used as a concrete additive.
Numerous processes for the removal of carbon have been proposed or employed. For example, carbon reduction in fly ash by electrostatic separation has been described in an article by David R. Whitlock, entitled "Electrostatic Separation of Unburned Carbon From Fly Ash" of International Separation Systems, Inc., incorporated by reference herein. The article proposes employing an electrostatic process involving contact charging separators. This is one of the two basic electrostatic processes for dry particle separation. A more conventional electrostatic process involves a rotating drum corona-type separator, but it is impractical for carbon reduction in fly ash.
Another means to separate carbon from fly ash by flotation with tall oil and kerosene has been attempted. However, this method has not proven economical due to the large capital equipment and operating expenses incurred in such processing. Mechanical screening and air classification methods have also met with only limited success. Relatively large amounts of fly ash is not suitable for processing according to these methods due to the methods' limitations on the size and weight distribution of fly ash that can be processed.
Another method is described in U.S. Pat. No. 5,390,611 to Richard E. John, which is incorporated by reference herein, where the fly ash is radiatively heated using electrical means in a rectangular channel and pulled through the channel by feed screws. A small volume of air and additional oxygen is fed into the channel through the feed screws to aid in ignition of the carbon in the fly ash. This method has met with only limited success due to low volumes of useable fly ash that is produced.
Other work has centered on the use of fluidized bed combustor means to remove carbon from fly ash. In the paper "Beneficiation of Fly Ash by Carbon Burnout" by J. S. Cochran and T. J. Boyd, incorporated by reference herein. This method suffers from high sensitivity to the variability in ashes from different sources, owing to significant differences in fluidization and mixing characteristics, auto-ignition temperatures, reaction times, and propensity to form sintered agglomerates. In addition, this approach advises the use of large and sophisticated air/handling and distribution systems that also employ gas particulate separators. The operation and control of the process involves precision and allows for little variability in the individual operating parameters without significant effect on others.
Similarly, U.S. Pat. No. 5,160,539 to Cochran, incorporated by reference herein, reveals a method and apparatus for burning carbon from fine particles of fly ash in a dry, bubbling fluid bed of previously introduced fly ash particles. The method also has a propensity for causing sintering and agglomeration of the fly ash, which is undesirable in fly ash to be used in concrete.
U.S. Patent No. 4,663,507 to Trerice, incorporated by reference herein, discusses a method and apparatus for reducing and measuring carbon content in fly ash that employs microwave energy to induce combustion of the carbon. U.S. Pat. No. 4,705,409 to Trerice, incorporated by reference herein, is a division of U.S. Pat. No. 4,663,507.
German Patent No. 2,611,213 to Steag AG, incorporated by reference herein, discusses making high quality clinker bricks from fly ash and clay, where the fly ash is initially roasted in the presence of a fuel such as coal dust or fuel gas. An external fuel source is used for the burnout operation.
Soviet Patent No. 734,162, incorporated by reference herein, discusses a process for thermally treating fly ash to reduce its carbon content which uses a solid heat carrier such as quartz sand and creates aggregates of fly ash, rather than fine particles.
Japanese Patent No. 57,179,067 to Kobe Steel KK, involves sintering of fly ash granules to form a lightweight aggregate, rather than fine particles. Japanese Patent No. 57,100,972 to Kobe Steel KK, also discusses a method for sintering fly ash granules.
The technology discussed in the above references leaves unsatisfied a need by industry for an apparatus and method for reducing carbon in fly ash that is simpler and more affordable to build and operate. Such an apparatus would, for example, eliminate the need for sophisticated control equipment and eliminate use of costly gas/particulate separators. An apparatus is also needed that solves the problem of sintering of the fly ash to form agglomerates without adversely affecting other physical properties of the fly ash for use in concrete. Further, an apparatus is needed that sufficiently meets the volume output requirements of the concrete industry for fly ash use as a pozzolonic admixture.